This invention relates to a communication system for communicating with a communication device which is connected to the communication system through a data bus and which is for use in communicating with an external device or a network.
It is known that a communication system comprises a plurality of interface circuits connected through a data bus to a communication device which is operable to control the communication system. The interface circuits communicate with the communication device through the data bus. On communicating with the communication device, the interface circuits employ or utilize the data bus in common. Accordingly, it is necessary to arbitrate employment or utilization of the data bus among the interface circuits.
In a conventional communication system, the interface circuits are connected to one another through a common bus which is different from a data bus. On employing the data bus, the interface circuits supply the common bus with individual addresses which are assigned to the interface circuits and which are different from one another. On the common bus, a logical OR signal appears which is representative of a logical addition of the individual addresses. When the logical OR signal is representative of a specific one of the individual addresses, the interface circuit of the specific address can employ the data bus to communicate with the communication device through the data bus.
Inasmuch as the employment of the data bus is decided in accordance with the individual addresses in the conventional communication system, it is difficult for the interface circuits to impartially employ or utilize the data bus.
In order to impartially employ the data bus among the interface circuits, an improved communication system is disclosed in Japanese Patent Prepublication No. 117543/1989, namely, Tokkai Hei 1-117543 under the title of "Access Control Device".
The improved communication system comprises a random number generator for generating a random number and an exclusive OR circuit for calculating an exclusive OR between the random number and each of the individual addresses to produce an exclusive OR signal. When a specific one of the individual addresses is coincident with the exclusive OR signal, a specific interface circuit having the specific address can employ the data bus to communicate with the communication device through the data bus.
However, it is difficult to quickly arbitrate employment of the data bus among the interface circuits since an arbitration is carried out by the use of the random number which is irregularly generated.